1. Field of the Invention
THE PRESENT INVENTION relates to an inflator and more particularly relates to an inflator for use in actuating and deploying a safety device, such as an air-bag, in a motor vehicle in the event that an accident should occur.
2. Description of Related Art
Many different types of inflator have been proposed previously for use in motor vehicles to actuate or deploy safety devices such as air-bags. In particular, it has been proposed to use gas generators which are termed “adaptive multi-stage inflators” which can be utilised to provide different inflationary effects in response to different types of crash or accident. In such an adaptive multi-stage inflator there may be two or more pyrotechnic charges that can be actuated in combination or independently. One pyrotechnic charge may be actuated before the other pyrotechnic charge, the delay before actuation of the second charge being 5 milliseconds or longer. A short delay may be appropriate for a very “hard” impact and a long delay may be more appropriate for a “light” impact. Many adaptive multi-stage inflators are of the “hybrid” type, meaning that the inflator has not only a pyrotechnic charge, but also a reservoir of compressed gas.
There are many important design parameters to take into account when designing an adaptive a multi-stage inflator, including the quantity, composition and configuration of the or each pyrotechnic charge, and the quantity, composition and pressure of the compressed gas if the gas generator is a hybrid gas generator. It is also necessary to provide an appropriate flow regulation for gas emerging from the inflator. Many pyrotechnic materials only provide an optimum “burn” characteristic when subjected to a back pressure and thus, in many cases, the flow of gas must be carefully regulated.
Thus, in some adaptive multi-stage inflators, where only a first pyrotechnic charge is to be actuated, the cross-section of the outlet flow path for gas can be relatively small, whereas if two or more pyrotechnic charges are actuated, the cross-section of the outlet flow path must be much greater. If two pyrotechnic charges are actuated within a relatively short period of time, the internal gas pressure may rise to very high levels, and if the cross-section of the outlet flow path is not appropriate, with regard to the quantity of gas being generated, then there is a risk that the parts of the inflator may be damaged.